Process and polycondensation apparatus for the treatment of contaminated ethylene glycol

ABSTRACT

The invention relates to a process for the treatment of contaminated ethylene glycol resulting during a polycondensation process for the production of polyester. In the course of this polycondensation process contaminated ethylene glycol is continuously predistilled, subjected as a distillation residue to secondary distillation, condensed and then returned to the polycondensation process as decontaminated ethylene glycol, so that an in-line recovery is achieved, which makes superfluous an external distillation installation for the treatment of contaminated ethylene glycol.

The invention relates to a process for the treatment of contaminatedethylene glycol which results during a polycondensation process for theproduction of polyester, wherein the contaminated ethylene glycol issubjected to preliminary distillation for separation of water and thelow-boiling fractions, then as a distillation residue is subjected tosecondary distillation and the ethylene glycol resulting as a distillateis condensed.

During the polycondensation process for the production of polyestercontaminated ethylene glycol results. So for instance a ethyleneglycol/water mixtures is formed in the esterification stage, during theesterification of terephthalic acid with ethylene glycol, formingpolyester. This ethylene glycol/water mixture is fed to a preliminarydistillation column, in order to eliminate water and other low-boilingfractions by distillation over the top of the preliminary distillationcolumn and to condense them in a subsequently arranged condenser. Forthe treatment of the contaminated ethylene glycol and consequently therecovery of the same ethylene glycol, the bottom product, respectivelythe distillation residue which consists of ethylene glycol, diethyleneglycol and other secondary products, have to be directed to an off-linedistillation apparatus, which consequently is not a part of thepolycondensation installation. Such an off-line distillation apparatusis expensive from the point of view of plant technology, requires inaddition considerable investments and is also disadvantageous from thepoint of view of space and service personnel requirements. In additionthereto the contaminated ethylene glycol resulting during thepolycondensation stage from the cycles of the spray condensers andvacuum pumps, as well as when the polycondensation apparatus is startedand closed down, has also to be fed to the off-line distillationapparatus, and as a consequence has to be decontaminated in the courseof an interrupted treatment process. The known treatment of contaminatedethylene glycol is unsatisfactory as a whole, from the point of view ofprocess technology as well as plant engineering.

It is the object of the invention to indicate a treatment process of theabove-described type, according to which the contaminated ethyleneglycol resulting during a polycondensation process can be perfectlydecontaminated in a rational, functionally safe and technologicallysimple way.

The invention solves this problem in the case of a generic process dueto the fact that during the polycondensation process the contaminatedethylene glycol is continuously predistilled, then subjected tosecondary distillation as a distillation residue, preferably undervacuum, then condensed and again fed to the polycondensation process,respectively its consumers, as decontaminated ethylene glycol. Accordingto the teaching of the invention an in-line vacuum distillation of theentire contaminated ethylene glycol resulting from the polycondensationprocess takes place. Indeed the ethylene glycol recovery is integratedinto the polycondensation process, namely by combining distillationprocess and treatment tasks. This way an external ethylene glycolrecovery becomes superfluous. The ethylene glycol treatment of theinvention leads to a particularly good cleansing, so that thedecontaminated ethylene glycol is particularly suitable for use inpolycondensation installations for producing high-quality polyesterproducts, such as granulate for the production of microfibers, foils,films and bottles. In fact the quality of fresh glycol is reached. Inthis connection it is particularly important that the secondarydistillation take place in vacuum, so that the usually high thermaloxidative load of the glycol is eliminated and an optimal productquality is achieved, namely the quality of fresh glycol. This waysavings of fresh glycol are generated.

Further steps which are essential to the invention are mentioned in thefollowing. So for instance the invention provides that the ethyleneglycol/water mixture resulting in the esterification stage or stagesduring the esterification of terephthalic acid with ethylene glycol toform polyester and the contaminated ethylene glycol resulting during thepolycondensation process from the cycles of the spray condensers andvacuum pumps be continuously treated to become decontaminated ethyleneglycol, which means it is subjected to preliminary distillation, thatthe distillation residue be subjected to secondary distillation undervacuum, the decontaminated ethylene glycol be condensed as a distillateand the recovered ethylene glycol be fed again to the polycondensationprocess, respectively to its consumers. Consequently a continuousrecovery is achieved also with respect to the contaminated ethyleneglycol resulting during the polycondensation stage. However, accordingto the invention also the contaminated ethylene glycol resulting fromthe starting, shutdown and cleaning processes of the polycondensationapparatus, after a temporary storage, is continuously fed to thecontinuous ethylene glycol recovery process, for instance is introducedin the esterification reactor. During the start, shut-off (cut-off) andcleaning of the polycondensation apparatus the so-called cleaning glycolis formed, which is actually also contaminated ethylene glycol. As aresult the treatment of the entire amount of ethylene glycol used andeventually contaminated in the polycondensation apparatus is achieved.

The object of the invention is also a polycondensation installation forthe production of polyester, with at least one esterification reactorand a subsequently arranged preliminary distillation column withcondenser for the preliminary distillation of the contaminated ethyleneglycol. This polycondensation installation which as a rule has twoesterification stages and one polycondensation stage is characterized inthat down from the bottom of the preliminary distillation column atransfer line for the distillation residue with a pressure-reducingdevice, e.g. a pressure-relief valve, leads to a secondary distillationcolumn, that at the top of the secondary distillation column a condenserfor the decontaminated ethylene glycol as a distillate is connected,that to the condenser on the one hand a return line with a feed pump fordecontaminated ethylene glycol, leading to the various consumers and onthe other hand vacuum line with a vacuum pump for producing a vacuum inthe secondary distillation column are connected, whereby from thepressure side of the vacuum pump a pressure line arranged before thefeed pump, considered in feeding direction, leads to the return lineunder conditions of pressure balancing. These steps taken according tothe invention make it possible to eliminate the off-line, thereforeexternal, distillation installation. Furthermore a continuouslyoperating recovery system is created which is integrated in the actualpolycondensation installation, which reduces considerably theinvestment, the building, personnel and energy expenses. Energy is savedalready because the polycondensation apparatus of the invention does notrequire a special heating of the distillation column, which is necessaryin an external distillation column during the recovery of ethyleneglycol. Besides an in-line installation is environmentally friendlierthan an off-line installation. Suitably to the preliminary distillationcolumn a supply line coming from the spray condensers and the vacuumpumps of the subsequently arranged polycondensation stage is connectedvia an intermediate container, which performs the function of a buffer,so that also the contaminated ethylene glycol resulting during thepolycondensation stage can be continuously recovered, without theassistance of external recovery systems. By means of the installedintermediate container it is possible to avoid load variations of thepreliminary distillation column. As a precautionary measure the transferline for the bottom product of the preliminary distillation column,respectively the distillation residue consisting of ethylene glycol,diethylene glycol and other secondary products has a pressure-relaxationtank, so that in the secondary distillation column it is indeed possibleto work with vacuum, respectively with sufficient negative pressure,while in the preliminary distillation column it is possible to work withthe usual operational pressure. In order to optimize the cleansing ofthe ethylene glycol in the secondary distillation column, the inventionfurther advises that between the condenser for the decontaminatedethylene glycol and the feed pump a reflux container be inserted in thereturn line and that from the reflux container a reflux line be providedwhich leads to the upper end of the secondary distillation column, wherea distribution device for the decontaminated ethylene glycol supplied byway of reflux is located. In fact a better separation of the ethyleneglycol from the low-boiling fraction is achieved this way. For thismerely the reflux of a small amount of decontaminated ethylene glycol issufficient. At the bottom of the secondary distillation columnpreferably an evacuation line with a feed pump for the low-boilingfraction is connected, so that the low-boiling fraction remaining at thebottom of the secondary distillation column can be evacuated andreleased for disposal by the operator of the installation. This way acareful separation of the low-boiling fraction--basically diethyleneglycol and other secondary products--is achieved. Further a supply linecoming from the storage container for contaminated ethylene glycol canopen into the esterification reactor, as a rule into the secondesterification reactor of the esterification stage. This because thisstorage container serves for receiving the contaminated ethylene glycolresulting during the start, the shut-off and the cleaning of thepolycondensation installation.

As a result the total amount of ethylene glycol circulating through thepolycondensation process is decontaminated without external treatmentand can be reused just like fresh glycol. Thereby the entirepolycondensation process is more economically structured. By limitingthe thermal-oxidative load of the decontaminated ethylene glycol throughthe secondary distillation in vacuum, the specification of pure,respectively fresh glycol is achieved in the course of an in-linedistillation.

The invention is closer described in the following with the aid of adrawing illustrating one embodiment example. The sole FIGURE shows thepart of a polycondensation apparatus equipped for the continuoustreatment of contaminated ethylene glycol.

The part of a polycondensation installation for the production ofpolyester shown in the FIG. 1 illustrates an esterification reactor 1 ofupstream esterification stage 2 not shown in the drawing, followed bythe polycondensation stage 3 also not shown in the drawing. Downstreamof the esterification reactor 1 a preliminary distillation column 4 withcondenser 5 is arranged. In the preliminary distillation column 4 waterand low-boiling fractions are separated through distillation from anethylene glycol/water mixture, condensed in condenser 5 and releasedinto the atmosphere, respectively discharged into the waste watertreatment system. From the sump, respectively bottom 6 of thepreliminary distillation column 4 a transfer line 7 for the distillationresidue of ethylene glycol, diethylene glycol and other secondaryproducts, provided with a pressure-reduction device 8, which is merelyindicated in the drawing, e.g. a pressure-relief valve, leads to asecondary distillation column 9. To the top of this secondarydistillation column a condenser 10 for the distillate, consequently forthe decontaminated ethylene glycol is connected. To the condenser 10, onthe one hand a return line 11 with a feed pump 12 for decontaminatedethylene glycol leading to the various consumers of the polycondensationinstallation and, on the other hand, a vacuum line 13 with a vacuum pump14 for creating a vacuum in the secondary distillation column 9 areconnected. From the pressure side of the vacuum pump 14 a pressure line15 arranged in feeding direction before the feed pump 12 leads to thereturn line 11 under conditions of pressure balancing. Thereby thedecontaminated ethylene glycol aspired by the vacuum pump 14 is fed tothe return line 11 for the rest of the decontaminated ethylene glycol.

A supply line 16 with an intermediate container 17, coming from thespray condenser and vacuum pumps of the downstream arrangedpolycondensation stage 3, is connected to the preliminary distillationcolumn 4. Thus this supply line 16 serves for feeding of decontaminatedethylene glycol resulting during the polycondensation stage 3. For therenonprecipitated vapors lead to an enrichment of the contaminatedethylene glycol in the vacuum pumps, which again is required for thepolycondensation stage 3 performed in vacuum.

As a precautionary measure, in the transfer line 7 between thepreliminary distillation column 4 and the secondary distillation column9, a pressure-relaxation tank 18 is inserted, in order to insure withcertainty the required pressure reduction between preliminarydistillation column 4 and the secondary distillation column 9. Betweenthe condenser 10 for the decontaminated ethylene glycol and the feedpump 12 a reflux container 19 is inserted in the return line 11. Fromthis reflux container 19 leads a reflux line 20 to the upper end of thesecondary distillation column 9, which has a spraying device 21 for thedecontaminated ethylene glycol. So far merely a small amount of thedecontaminated ethylene glycol is supplied to the secondary distillationcolumn 9 by way of the reflux, in order to enhance the cleansing effectin there. An evacuation line 23 with a feed pump 24 for the high-boilingfractions is connected to the bottom 22 of the secondary distillationcolumn 9.

Into the esterification reactor 1 leads a supply line 25 coming from astorage container, for contaminated ethylene glycol resulted during thestart, shut-off and cleaning of the polycondensation installation.

We claim:
 1. Process for the treatment of contaminated ethylene glycolresulting from a polycondensation process for the production ofpolyester, according to which contaminated ethylene glycol is subjectedto preliminary distillation for separation from water and low-boilingfractions, as a distillation residue is subjected to secondarydistillation and the ethylene glycol as a distillate is condensed,characterized in that in the course of the polycondensation processcontaminated ethylene glycol is continuously subjected to preliminarydistillation, subjected to secondary distillation as a distillationresidue, under vacuum, condensed and as decontaminated ethylene glycolresupplied to the polycondensation process.
 2. Process according toclaim 1, characterized in that the ethylene glycol/water mixtureresulting from the esterification stage or stages of terephthalic acidwith ethylene glycol to form polyester and the contaminated ethyleneglycol resulting from the cycles of the spray condensers and vacuumpumps are continuously treated during the polycondensation process tobecome decontaminated ethylene glycol, by way of preliminarydistillation, secondary distillation in vacuum and condensation. 3.Process according to claim 1, characterized in that the contaminatedethylene glycol resulting from the start, shut-off and cleaningprocesses of the polycondensation installation, after a temporarystorage, is continuously reintroduced to the continuous ethylene glycolrecovery process.